In the final stage of music production an objective listener in a controlled listening environment is of great use to the artist, the producer and the mixing engineer.
The mastering process starts with a dialogue where we establish what your goals and musical intentions are.
Next, you send me your mixes along with reference tracks, perhaps songs that have inspired you or songs with a sound similar to what you're aiming for. Reference tracks are a useful way of describing your intentions. I listen to your mixes and offer feedback. Sometimes I suggest mix revisions.
Once the mixes are ready for mastering, I make sure that the record translates as well as possible across a wide range of playback systems; that there's sonic cohesion between songs; and that the artistic vision comes across as clearly and directly as possible, without unwanted noise, frequency build-ups or other issues that could detract from the listening experience.
The primary mastering tools, apart from a good set of speakers in a good room, are equalizers, limiters and compressors.
When I'm happy with the sound I'll send you either 16 or 24 bit WAV files depending on how your music will be distributed. After listening to the mastered tracks, preferably wherever you usually listen for pleasure, you get back to me with feedback and I do revisions if needed. A few revisions are part of the deal.
Once you're happy with the WAV files I can provide you with other file formats as well.
In the end, mastering is a service job. I'm done when the client is happy. Communication with the client is at the heart of what I do.